Lithium niobate, LiNbO.sub.3, is a well known nonlinear optical material utilized for its electrooptic, ferroelectric and piezoelectric properties. LiNbO.sub.3 is widely used in integrated and guided-wave optics applications, such as couplers, switches, modulators, deflectors and rf spectrum analyzers. Optical waveguiding devices typically are composed of two layers having different refractive indices. One common optical waveguide combination is a layer of LiNbO.sub.3 disposed over a sapphire substrate. Sapphire (single crystal alumina) is an attractive substrate for LiNbO.sub.3 because of its much lower refractive index and relatively low cost.
Although polycrystalline and single crystal LiNbO.sub.3 have been prepared by a variety of standard solid state synthetic methods, relatively little work has been reported dealing with the fabrication of thin films of this material. Magnetron and rf sputtering and molecular beam epitaxy have been used to fabricate thin films of LiNbO.sub.3. These techniques, however, require complicated and expensive high vacuum equipment. It is also difficult, using these processes, to maintain film uniformity over large areas. Further disadvantages are the low growth rates associated with these techniques as well as their difficulty in providing conformal coverage. Conformal coverage is the degree to which the deposition follows the contour of the substrate surface. Line of sight techniques such as sputtering typically exhibit poor conformal coverage.
In "Preparation of Crystalline LiNbO.sub.3 Films with Preferred Orientation by Hydrolysis of Metal Alkoxides," Advanced Ceramic Materials, Vol. 3, No. 5, 1988, a process is disclosed for preparing LiNbO.sub.3 thin films using single source reagents by a sol gel process utilizing hydrolysed metal alkoxides, such as, for example, an ethanol solution of lithium ethoxide and niobium ethoxide. It is difficult to obtain epitaxial films using sol-gel deposition techniques as it is not possible to control the crystallization rate using this process.
U.S. Pat. No. 5,051,280 discloses a method for producing alkali metal niobates and tantalates which involves pyrolyzing the stoichiometric salt of an alkali metal and a niobium or tantalum complex of a bidentate or tridentate ligand. For example, the salt may be dissolved and then coated on a substrate, after which the coating is pyrolysed at 400.degree. C. for 10 minutes. It is difficult to obtain high quality films having monocrystallinity using such a organometallic decomposition method, due to the lack of control in the crystallization rate as well as the relatively large amount of organic material that is evolved during the processing of the coated precursor layer.
Chemical vapor deposition (CVD) is a technique capable of depositing films at high growth rates. To date there has been little work done on depositing complex metal oxide films such as LiNbO.sub.3 by CVD.
"Metal Alkoxides as Precursors for Electronic and Ceramic Materials", by Bradley, Chem. Rev., Vol. 89, pages 1317-1322 (1989), mentions that various metal alkoxides offer several advantages (such as the ability to purify them to a high degree) as precursors for the deposition of metal oxides. However, Bradley teaches us that, while deposition of binary metal oxides like ZrO.sub.2 are comparatively easy, difficulties arise when ternary or quaternary heterometal oxides are required. According to Bradley, lithium niobate is illustrative of the problem, i.e., although LiNb(OR).sub.6, (in particular LiNb(OBu.sup.t).sub.6), has the correct stoichiometry for depositing LiNbO.sub.3, the volatilization of LiNb(OR).sub.6 sometimes leads to some disproportionation and the disparity in volatilities of the parent alkoxides results in nonstoichiometric films.
In other words, the attempted volatilization of LiNb(OBu.sup.t).sub.6 results in the generation of the constituent monometallic alkoxides which have different volatilities. According to Bradley, Curtis and Brunner appear to have solved this problem with a two source process using the lithium derivative of a beta diketone and a niobium alkoxide. However, the method used by Curtis and Brunner, disclosed in U.S. Pat. No. 3,911,176 to Curtis et. al., and in "The Growth of Thin Films of Lithium Niobate by Chemical Vapor Deposition," by B. Curtis and H. Brunner, Mat. Res. Bull. Vol. 10, pp. 515-520, 1975, teaches vaporizing separate lithium and niobium compounds and afterward bringing the resultant vapors together and in contact with a heated substrate. Preferred materials for producing the vapor precursor include lithium chelates of beta-diketonates and niobium alkoxides. One disadvantage to a process such as this, which utilizes separate sources for each metal precursor, is that the system is very difficult to control because each precursor volatilizes at a different rate. Consequently, because there is usually more of one metal than the other in the precursor vapor, it is difficult to achieve the desired 1:1 stoichiometry in the resultant LiNbO.sub.3 film. In addition, when dual source precursors are involved, it is likely that one of the metals will deposit preferentially, leading to gradient film compositions. Further, the films deposited using the Curtis CVD method are initially black, and therefore not useful in many electro-optic applications which require high light transmittance, until they have first been annealed. Curtis teaches that these films should be annealed at a temperature of about 700.degree. to 1000.degree. C. for 1 to 10 hours. Finally, as stated in "The Growth of Thin Films of Lithium Niobate by Chemical Vapor Deposition", the Curtis method is not suitable for depositing on many suitable electro-optic substrates, including sapphire.
Although epitaxial LiNbO.sub.3 films have been produced using a variety of deposition techniques, such as liquid phase epitaxy (LPE), sputtering, and laser ablation, the LiNbO.sub.3 films formed using these techniques had several disadvantages.
LiNbO.sub.3 films prepared using LPE techniques have the disadvantage of requiring a very high deposition temperature. Because of this high processing temperature, the substrate must be the same as the material to be deposited. Otherwise, interaction between the substrate and the film will occur. For example, if LiNbO.sub.3 were attempted to be grown on Al.sub.2 O.sub.3, Li would tend to diffuse into the Al.sub.2 O.sub.3 substrate to form lithium aluminates. Thus, LPE cannot be used to deposit LiNbO.sub.3 on substrates other than LiNbO.sub.3, such as, for example on sapphire, LiTaO.sub.3, silicon, GaAs, etc. In addition, LPE processes are not capable of preparing thin films (1 micrometer thick or less) having a uniform film thickness. For example, using LPE, it is difficult to achieve thickness deviations below 20 percent on 20 sq cm surface area substrates (3.14 sq. in.).
Films prepared by sputtering are not of a truely single crystalline quality, but rather have some defects which adversely affect electro optic performance.
Thus, there continues to be a need for a chemical vapor deposition process that will produce monocrystalline niobate or tantalate films, such as, for example, lithium niobate, which preferably have an epitaxial relationship to the substrate and do not require a subsequent annealing operation to produce optically transparent films.
There also continues to be a need for a process which can deposit useful LiNbO.sub.3 films on a wide variety of substrates. Also, during mass production of LiNbO.sub.3, it is desireable to be able to deposit thin (less than 1 micrometer) uniform thickness films on circular substrates having a diameter of at least 5.08 cm (2 inches), more preferably at least 20.32 cm (8 inches). Other shaped substrates having similar surface areas (20.3 sq. cm (3.14 sq. in.) more preferably at least 324 sq cm (50 sq. in.), could also be used. Then, if need be, the larger substrate can be cut into smaller pieces.